Abyssal harzburgite veined by silica-oversaturated melt in the Sibuyan Ultramafics, Romblon, Central Philippines

Abstract

Thin discordant gabbronorite veins (~0.5 mm to 3 cm in width) occur within remarkably fresh harzburgite boulders of the Sibuyan Ultrarnafics. The harzburgite host rock displays protogranular to porphyroclastic textures and is dominantly composed of olivine. orthopyroxene with minor amounts of clinopyroxene, spinel and am-plubole. The mineral chemistry of the harzburgite is comparable to depleted abyssal perklotites as shown by the Fo content of the ohvine (= 90-91) and Cr# of the spinel (= 0.40-0.52). The gabbronorites are coarse-grained adcumulates and comprised of orthopyroxene, plagioclase and amphibole. Plagioclase hi the gabbronorites shows high An content whereas both oithopyroxene and amphibole show variable Mg# almost similar to reported values from arc gabbros. The contact between the harzburgite and the gabbronorite veins is demarcated by the foimation of secondary orthopyroxene with low Cr2O3 and CaO contents. Clinopyroxetie in the harzbur-gite shows strong enrichment in light rare earth elements (LREEs). Amphibole rimming the clinopyroxene in the harzburgite has similar patterns as the clinopyroxene but with much higher REE abundance. The amphibole in the gabbronorites also shows enrichment in LREEs, Rb, Ba and Ti. We propose that the harzburgite-gab-bronorite occurrence in the Sibuyan Ultrarnafics is a product of mantle-melt reaction. The metasornatic agent is a silicate melt enriched in Si, Cr, Fe and LREEs. At a bigger scale, the harzburgite-gabbionoiite connection observed in the Sibuyan Ultramafics possibly documents early stage modification and conversion of abyssal peri-dotites to ophiolitic peridotites by SSZ-related melts.